Miniature hearing devices are commonly powered by mercury-free zinc air button cell batteries. Mercury-free zinc air battery cells suffer from gassing, which happens after deep discharge when the battery stays (e.g. is forgotten) in the battery compartment of a hearing device for a prolonged period of time. This may lead to battery leakage. Therefore, deep discharge of mercury-free battery cells should be prevented.
Some known hearing devices may be switched off by disconnecting the battery from the electronic circuitry. This can for instance be achieved by opening the battery door, thereby releasing the battery from the electrical battery contacts. In this way discharging of the battery is minimised.
Some other known hearing devices may enter into a “powerdown” state upon the user issuing an “off” command, such as for instance by flipping an on/off switch. In this off/powerdown state certain triggers are still monitored by the hearing device, which may then lead to waking up the hearing device again, e.g. by the user issuing an “on” command, for instance by actuating a user control element. In order to monitor wake-up triggers at least some of the electrical circuitry of the hearing device must be supplied with power, even when the hearing device is not actually being used. The hearing device may automatically enter into a “shutdown” state once the battery gets weak. The residual current in the shutdown state may result in battery deep discharge and gassing, when for instance the battery cell is forgotten in the hearing device.
Known hearing devices employ power supply coupling or buffer capacitors. The buffer capacitor for instance absorbs loudspeaker current peaks and stabilises the whole electrical system of the hearing device. Furthermore, the buffer capacitor provides a continual power supply during phases when the battery is briefly disconnected from the electrical battery contacts, for instance when the battery compartment or door is opened for a short amount of time (e.g. to replace the battery) or as a result of power glitches, i.e. short interruptions of the battery supply voltage due to mechanical vibrations or shocks. The provision of a large buffer capacitor can however result in unwanted deadlock situations, where the hearing device will not perform a power on reset (POR) and remain inoperable despite replacing a depleted battery with a new one, leading to confusion and frustration of the user. The latter problem is especially acute in the case where the residual drain current in the shutdown state is minimised in order to avoid deep discharging of a battery, especially of a mercury-free battery. The stated deadlock situation can occur when the supply voltage is required to make a transition from below across a pre-set threshold value in order to trigger a POR and switch the hearing device from the shutdown state into a normal operation state. In case the buffer capacitor is discharged very slowly because the residual drain current is very small, the supply voltage may not have dropped below the threshold value by the time the battery is replaced and therefore the required transition will not occur, and hence no POR will be triggered.